1. Field of the Invention
The present invention relates generally to a class of field effect current control devices, and relates more particularly to rectifiers and current limiting devices constructed in the III-Nitride material system.
2. Description of Related Art
III-nitride semiconductors are presently known that exhibit a large dielectric breakdown field of greater than 2.2 MV/cm. III-nitride heterojunction structures are also capable of carrying extremely high currents, which makes devices fabricated in the III-nitride material system excellent for power applications.
Development of devices based on III-nitride materials has generally been aimed at high power-high frequency applications such as emitters for cell phone base stations. The devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFETs), high electron mobility transistors (HEMTs) or modulation doped field effect transistors (MODFETs). These types of devices are typically able to withstand high voltages such as in the range of 100 Volts, while operating at high frequencies, typically in the range of 2-100 GHz. These types of devices may be modified for a number of types of applications, but typically operate through the use of piezoelectric polarization fields to generate a two dimensional electron gas (2DEG) that allows transport of very high current densities with very low resistive losses. The 2DEG is formed at an interface of AlGaN and GaN materials in these conventional III-nitride HEMT devices. Due to the nature of the AlGaN/GaN interface, and the formation of the 2DEG at the interface, devices that are formed in the III-nitride materials system tend to be nominally on, or depletion mode devices. The high electron mobility of the 2DEG at the interface of the AlGaN/GaN layers permits the III-nitride device, such as a HEMT device, to conduct without the application of a gate potential. The nominally on nature of the HEMT devices previously fabricated have limited their applicability to power management. The limitations of nominally on power devices is observed in the need to have a control circuit be powered and operational, before power can be safely controlled by a III-nitride HEMT device. Accordingly, it would be desirable to create a III-nitride HEMT device that is nominally off to avoid current conduction problems during start-up and other modes.
A drawback of III-nitride HEMT devices that permit high current densities with low resistive losses is the limited thickness that can be achieved in the strained AlGaN/GaN system. The difference in the lattice structures of these types of materials produces a strain that can result in dislocation of films grown to produce the different layers. This results in high levels of leakage through a barrier layer, for example. Some previous designs have focused on reducing the in-plane lattice constant of the AlGaN layer to near where the point of relaxation occurs to reduce the dislocation generation and leakage. However, the problem of limited thickness is not addressed by these designs.
Another solution is to add insulation layers to prevent leakage problems. The addition of an insulator layer can reduce the leakage through the barrier, and typical layers used for this purpose are silicon oxide, silicon nitride, sapphire, or other insulators, disposed between the AlGaN and metal gate layers. This type of device is often referred to as a MISHFET and has some advantages over the traditional devices that do not have an insulator layer.
While additional insulator layers can permit thicker strained AlGaN/GaN systems to be constructed, the confinement layer produced by the additional insulator results in lower current carrying capacity due to the scattering effect produced on electrons at the GaN/insulator interface. Also, the additional interface between the AlGaN layer and the insulator results in the production of interface trap states that slow the response of the device. The additional thickness of the oxide, plus the additional interfaces between the two layers, also results in the use of larger gate drive voltages to switch the device.
Conventional device designs using nitride material to obtain nominally off devices rely on this additional insulator to act as a confinement layer, and may reduce or eliminate the top AlGaN layer. These devices, however, typically have lower current carrying capacity due to scattering at the GaN/insulator interface.
Accordingly, it would be desirable to produce a heterojunction device or FET that has a low leakage characteristic with fewer interfaces and layers that can still withstand high voltage and produce high current densities with low resistive losses. Presently, planar devices have been fabricated with GaN and AlGaN alloys through a number of techniques, including MOCVD (metal organic chemical vapor deposition) as well as molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
Materials in the gallium nitride material system may include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN). These materials are semiconductor compounds that have a relatively wide direct bandgap that permits highly energetic electronic transitions to occur. Gallium nitride materials have been formed on a number of different substrates including silicon carbide (SiC), sapphire and silicon. Silicon substrates are readily available and relatively inexpensive, and silicon processing technology has been well developed.
However, forming gallium nitride materials on silicon substrates to produce semiconductor devices presents challenges that arise from differences in the lattice constant, thermal expansion and bandgap between silicon and gallium nitride. The problems attendant with the lattice mismatch between GaN and traditional substrate materials are also prevalent in material layer structures involving GaN and GaN alloys. For example, GaN and AlGaN materials have lattice structures that differ significantly enough to produce interface strain between the layers, contributing to piezoelectric polarization. In many previous devices, the fields generated by the piezoelectric polarization are controlled to improve the characteristics of the devices. Variations in the content of aluminum in the AlGaN/GaN layer structures tends to vary the lattice mismatch between the materials to achieve different device characteristics, such as improved conductivity or isolation barriers.
A particular application that would benefit from a semiconductor structure with a low forward resistance or voltage drop is a rectifier. Conventionally, rectifiers in power applications are often controlled as synchronous rectifiers, where a diode performs the voltage blocking function of the rectifier, and a synchronous switch across the diode performs current conduction when the diode is forward biased to avoid the ON resistance and forward voltage drop of the diode. This type of configuration requires a switching control for the switch so that it is turned on when the diode is to be operated in forward conduction mode. This type of conventional synchronous rectifier is often used in power applications to avoid the drawbacks of diodes used as rectifiers, such as a forward voltage drop that reduces power efficiency and may also contribute to thermal operating issues. However, because the synchronous switch used with the diode to construct a synchronous rectifier is typically a power switch, a gate driver is used to operate the switch in high power applications. It would therefore be desirable to obtain a rectifier device that does not require additional control operations, but also maintains a low forward voltage drop.
Another type of device that can limit the amount of current flowing through it is a pinch resistor. A pinch resistor is typically fabricated in a semiconductor material with two different types of conductivity material. For example, a P-type conductivity material is disposed in an N-type region, and an N+-type region is formed partially over the P-type material. The resistance of the pinch resistor is determined by the sheet resistance of the P-type material under the overlapping N+-type region and the dimensions of the overlapped P-type material. The current in the device is conducted in a channel that is “pinched” by the opposite semiconductor material. Through these types of designs, very high resistances can be formed in a very small area to effectively limit current in a given power range. However, these types of devices are often imprecise due to processing techniques used during their manufacturing. According to some designs, the current through the device is effectively limited by the voltage generator across the device, which can serve to control the channel to limit the current. In conventional devices, the current values that can be carried in pinch resistors is relatively low, so that power applications tend to be somewhat complicated.
It would be desirable to provide a high current rectifier device such as a diode or pinch resistor that can block significant voltages while obtaining a low forward voltage drop.